The present invention relates to relay systems for use in protecting AC power distribution systems and more particularly to out-of-step blocking units for preventing circuit breaker operation as a result of load swings.
Sometimes, as a result of anomalous operation of the power system, for example when the system is not operating in synchronization, the impedence characteristics will change and, in some cases, change to the extent that the protective relay system treats such a change as a fault condition thereby generating a trip signal to the circuit breaker protecting the line. Since it may be undesirable to trip the circuit breakers under these conditions, out-of-step blocking units have been incorporated into protective relay systems to provide a blocking signal which prevents circuit breaker tripping as a result of load swings.
Load swing conditions can be visualized by referring to the R-X diagram shown in FIG. 1. The path of the impedence point Z caused by a load swing is represented by the line 10. As can be seen, the path of the impedence point enters the characteristic of an overreaching relay, represented by circle 12, as well as the characteristic of a first zone relay, represented by the circle 14. As is known in the art of protective relaying, once the load impedance comes within the characteristic of a protective relay, the relay will generate a trip signal. For the condition depicted in FIG. 1, the overreaching relay will generate a trip signal once the load point comes within its characteristic 12; and the first zone relay will generate a trip signal once the load point enters its characteristic 14.
As indicated above, it is undesirable that the circuit breakers trip on an impedence change caused by a load swing. Consequently, it is desirable to be able to discriminate between a load swing and a fault condition. This has been accomplished by adding an out-of-step blocking unit, which is essentially an overreaching relay, to the system. Referring to FIG. 2, there is shown a R-X diagram on which is depicted the characteristic of the out-of-step blocking unit, represented by circle 16, as well as the characteristics of the overreaching relay, represented by the circle 12 and the first zone relay, represented by the circle 14.
It has been found that a change in impedence due to a swing causes the impedence to change at a rate which is slower than that which occurs as the result of a fault. As a result, load swings, for which tripping is not desired, are detected by measuring the time from which the impedence enters the characteristic of the out-of-step blocking relay, circle 16 in FIG. 2, and the time the impedence comes within the characteristic of the overreaching relay, circle 12 in FIG. 2, or the first zone relay, circle 14 in FIG. 2. If this time difference exceeds a predetermined limit, the system treats the change in impedence as a load swing and therefore causes the out-of-step blocking unit to generate a blocking signal thereby preventing the tripping of the circuit breakers.
One problem with prior out-of-step blocking units is that they can generate blocking signals under certain fault conditions in which tripping should be allowed to occur. For instance, a high resistance ground fault may cause the impedance seen by a relay at one end of the protected line to fall between the tripping characteristic and the out-of-step blocking characteristic, thus causing the out-of-step circuit to block tripping. This adversely affects the reliability of the system by preventing the operation of the circuit breakers when they should be tripped to isolate a fault.
Another problem experienced in the past has been a lack of coordination between the out-of-step blocking unit and the tripping units on internal faults. In some instances, the blocking unit will operate prematurely with respect to the tripping units once again causing circuit breaker operation to be erroneously blocked.
Accordingly, it is an object of the present invention to provide a protective relay system for protecting AC power transmission lines incorporating an out-of-step blocking unit, which system has enhanced reliability.
It is another object of the present invention to provide a protective relay system for an AC power transmission line in which the coordination of the out-of-step blocking unit and the tripping units is enhanced.
It is still another object of the present invention to provide a protective relay system for an AC power transmission line in which coordination between blocking units and tripping units is enhanced by preventing or substantially delaying the operation of the out-of-step blocking unit on internal faults.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.